<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>

<body style="background: rgba(199,237,204,1)">

    <div style="display:flex; flex-direction: row">

        <!--通过style方式为canvas设置宽高在火狐浏览器上导致绘制内容纵向拉伸。。。-->
        <canvas id="drawing" width="400" height="400">canvas to draw</canvas>

        <pre id="container" style="margin: 10px" />

        <img src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1494089659713&di=2de3a402d0a89cd3e5ea785e30150928&imgtype=0&src=http%3A%2F%2Fsh.sinaimg.cn%2F2014%2F0715%2FU3551P18DT20140715192106.jpg">
    </div>
</body>
<script type="text/javascript">
    window.onload = function () {
        var drawing = document.getElementById('drawing');

        if (drawing.getContext) {
            print('support')
            addRoundRectFunc();
            var context = drawing.getContext('2d');
            draw(context);

        } else {
            print('not ')
        }
    }


    function draw(context) {
        context.fillStyle = 'red';

        var image = document.images[0];

        context.roundRect(0, 0, image.width / 2, image.height / 2, 30, true)

        context.globalCompositeOperation = 'source-in';
        context.drawImage(image, 0, 0, image.width / 2, image.height / 2)
        // toImage();  

    }

    function addRoundRectFunc() {
        CanvasRenderingContext2D.prototype.roundRect =
            function (x, y, width, height, radius, fill, stroke) {
                if (typeof stroke == "undefined") {
                    stroke = true;
                }
                if (typeof radius === "undefined") {
                    radius = 5;
                }
                this.beginPath();
                this.moveTo(x + radius, y);
                this.lineTo(x + width - radius, y);
                this.quadraticCurveTo(x + width, y, x + width, y + radius);
                this.lineTo(x + width, y + height - radius);
                this.quadraticCurveTo(x + width, y + height, x + width - radius, y + height);
                this.lineTo(x + radius, y + height);
                this.quadraticCurveTo(x, y + height, x, y + height - radius);
                this.lineTo(x, y + radius);
                this.quadraticCurveTo(x, y, x + radius, y);
                this.closePath();
                if (stroke) {
                    this.stroke();
                }
                if (fill) {
                    this.fill();
                }
            };
    }

    function toImage() {
        var imageUri = drawing.toDataURL('image/png');
        var imageTag = document.createElement('img');
        imageTag.style = 'margin:10px;width:200px;height:200px'
        imageTag.src = imageUri;
        document.body.appendChild(imageTag)
    }

    function print(txt) {
        document.getElementById("container").innerHTML += ('\n') + txt;
    }

    document.body.onclick = function () {
        window.location.reload()
    }
    console.log = print;
</script>


</html>